The present invention is directed to an electroplated and painted steel substrate which exhibits improved corrosion and paint delamination resistance.
Steel has been known and used for years as a construction product However, an accepted condition of such use, depending on the environment to which the steel was exposed, was that the steel was subject to corrosive attack. In the desire to minimize such attack, workers in the art sought out methods and protective coatings for the steel. Today, zinc is one of the most widely used metallic coatings applied to steel surfaces to protect them against corrosive attack. Two principal methods of applying such coatings are (1) hot-dipping, and (2) electroplating. Hot-dipping has the advantage of cost and ease of application. However, hot-dipping typically results in a thick coating with a rough surface, and an intermetallic alloy interface between the steel substrate and coating overlay. As a consequence, the formability and appearance of hot-dip products is limited, thus making such product unacceptable for many applications.
In contrast, electroplated zinc (1) produces smooth, thinner coatings, (2) is applied at lower temperatures, which means the base steel is less affected by such temperatures, and (3) results in little or no formation of an intermetallic alloy interface. Thus, where forming is a critical step in the fabrication of a product, electroplated zinc is the preferred product.
Zinc, when applied as a thin electroplated coating to steel, offers only minimum protection against corrosion. This shortcoming of pure zinc led to further research to improve the corrosion resistance of electroplated coatings. In addition, at points where there are breaks in the coating down to the base steel, extensive corrosion of the zinc coating under the paint film (layer) occurs, which causes severe paint delamination.
Shanz, in U.S. Pat. No. 2,419,231, teaches that a zinc electroplated coating, containing nickel, improves the corrosion resistance of the coating layer. The Ni-Zn alloy compositions suggested by Shanz contain 10 -24% Ni, balance Zn. A preferred feature of the Shanz product is the application of a pure nickel layer on the steel prior to the electrocoating with Ni-Zn.
Subsequent developments, such as described in the patents to Roehl, No. 3,420,754; Roehl, et al., No. 3,558,442; and Hirt, et al., No. 4,282,073, have generally sought to further improve the corrosion resistance through changes or controls imposed on the coating practices, and/or changes to the coating composition. None, however, have addressed themselves to the problem and solution of resistance to paint delamination. Applicant will discuss the latter in the specifications which follow.